This invention relates to a porous membrane having both a hydrophobic (water repellent) and oleophobic (oil repellent) surface. More particularly, this invention relates to a microporous or ultrafiltration membrane modified to produce a hydrophobic/oleophobic surface including the membrane pore surfaces and to a process for forming such a membrane.
Polytetrafluoroethylene (PTFE) has been the most commonly used material in membranes utilized to vent gases. The chemical and biological inertness, thermal stability, and hydrophobicity inherently associated with PTFE has led to the development of PTFE as the material of choice in industrial gas vent applications. PTFE membranes have also found widespread use in the health and related industries. The necessity of producing aseptic vent membranes for use in medical/biological devices has also naturally led to the selection of PTFE as the choice material in membrane applications. Traditionally, aseptic materials have been generated by chemical sterilization, notably by steam treatment or treatment with ethylene oxide. The compatibility of PTFE with sterilizing chemicals and treatments, especially at elevated temperatures, is a well known material property characteristic of PTFE. A problem sometimes encountered with the use of PTFE as a vent membrane material under steam treatment is pore blockage either due to condensation of oil, from the machinery used to generate the steam, or water or both. The resulting loss of air permeability of the clogged membrane effectively reduces the membrane's utility as a gas vent. This condensation problem has led to the search and development of more hydrophobic and oleophobic membrane materials as substitutes for PTFE. A more acute problem concerns the chemical sterilization of membrane materials for use under aseptic conditions. Chemical sterilization, particularly with ethylene oxide, very often generates additional issues such as toxicity and waste disposal that raises serious health, environmental and economic concerns. These concerns have led to the widespread use of ionizing radiation for sterilization of materials used in medical and biological devices. A major disadvantage of PTFE is its inherent instability towards ionizing irradiation. Ionizing irradiation of PTFE membranes results in the undesirable property of reduced mechanical strength. This loss of mechanical strength places severe restrictions in the use of PTFE membranes under moderate pressures.
Coating of materials allows one to retain the desirable bulk materials properties while only altering the surface and interfacial properties of the substrate. Hydrophobic and oleophobic coatings have found popular use in the electronics industry as protective barriers for sensitive electronic components. Although expensive, coating solutions are commercially available. Coating membranes has not been a very practical approach for modifying the surface properties of membranes since the tortuous morphologies associated with membranes rarely lend themselves to a continuous and even coating. Furthermore, since coatings are not permanently anchored (bonded) to the underlying substrate, very often the coated materials are susceptible to wear and extraction thereby having a rather limited range of thermal and chemical compatibility. In addition, coatings adversely affect the permeability properties of porous substrates.
It also has been proposed to utilize grafting techniques to modify the surface characteristics of a polymer substrate. Typical examples of grafting techniques are shown, for example, in U.S. Pat. Nos. 3,253,057; 4,151,225; 4,278,777 and 4,311,573. It is difficult to utilize presently available grafting techniques to modify the surface properties of porous membranes. This is because it is difficult to modify the entire surface of the membrane including the surfaces within the pores while avoiding pore blockage and while retaining membrane porosity.
It has been proposed in U.S. Pat. No. 4,954,256 to render the surface of a microporous polymeric membrane more hydrophobic by grafting a fluoropolymer to the membrane surface in order to chemically bond the fluoropolymer to the membrane surface. The fluoropolymer is formed from a monomer containing an ethylenically unsaturated group and a fluoroalkyl group. The grafting is effected by exposing the membrane, in a monomeric solution, to ionizing radiation. A typical source of ionizing radiation is a .sup.60 Co gamma radiation source. The fluoropolymer formed from the fluorine-containing ethylenically unsaturated monomer is permanently bonded to the microporous membrane substrate.
European patent application 86307259.1 discloses a process for preparing hydrophobic/oleophobic membranes. The process is not a surface modification; it is an in situ process which, by virtue of a phase separation, both the underlying substrate and hydrophobic surface of the membrane are formed simultaneously by a photopolymerization process. The resulting membrane is weak mechanically and needs to be supported/laminated for use as a vent membrane under relatively moderate pressures. In addition, the process gives rise to membranes with a relatively narrow range of properties since the membrane morphology and surface characteristics are formed simultaneously.
Patent application PCT/US90/04058 discloses a process for preparing hydrophobic and oleophobic porous substrates. The process entails impregnating a porous substrate with a solution of a fluorinated monomer in a carrier solvent, removal of the solvent by evaporation, and then polymerization of the remaining monomer. The process is, in essence, a solid-state polymerization reaction.
U.S. Pat. No. 4,618,533 discloses a process for forming a composite membrane from a porous membrane substrate and a cross-linked, polymerizable monomeric composition coated on the substrate. The monomeric composition includes a polymerizable monomer and a cross-linking agent for the monomer. Any conventional energy source for initiating free radical polymerization can be used to form a cross-linked polymeric coating in situ on the porous membrane such as ultraviolet (UV) light or heat. By this process, a membrane having its surface modified by the cross-linked polymer is produced. No mention is made of forming a cross-linked modified surface from an ethylenically unsaturated monomer having a fluoroalkyl group.
U.S. Pat. No. 5,037,457 discloses a means for enhancing the mechanical strength of gamma irradiated PTFE membranes by laminating the PTFE membrane to a porous polyester web. This approach resolves the practical issue concerning the mechanical stability of gamma irradiated PTFE. The chemical compatibility of the laminated membrane is now limited by the properties of the porous web support. Furthermore, laminates are prone to delamination, particularly laminates formed by the use of adhesives which often are sensitive to gamma radiation.
Accordingly, it would be desirable to provide a porous membrane having a surface which is more hydrophobic and oleophobic than presently available membranes. In addition, it would be desirable to provide such a membrane which retains its mechanical strength after being exposed to sterilizing ionizing radiation.